replace fuchsia-zircon with fuchsia-cprng in rand-0.4

As in #706, fuchsia is replacing users of fuchsia-zircon with
fuchsia-cprng since that reduces exposure to our changing syscalls.

Author: erickt

Cargo.toml

@@ -32,7 +32,7 @@ winapi = { version = "0.3", features = ["minwindef", "ntsecapi", "profileapi", "
 members = ["rand-derive"]
 
 [target.'cfg(target_os = "fuchsia")'.dependencies]
-fuchsia-zircon = "0.3.2"
+fuchsia-cprng = "0.1.0"
 
 [target.'cfg(target_env = "sgx")'.dependencies]
 rdrand = "0.4.0"

src/os.rs

@@ -389,7 +389,7 @@ mod imp {
 
 #[cfg(target_os = "fuchsia")]
 mod imp {
-    extern crate fuchsia_zircon;
+    extern crate fuchsia_cprng;
 
     use std::io;
     use Rng;
@@ -413,15 +413,7 @@ mod imp {
             next_u64(&mut |v| self.fill_bytes(v))
         }
         fn fill_bytes(&mut self, v: &mut [u8]) {
-            for s in v.chunks_mut(fuchsia_zircon::sys::ZX_CPRNG_DRAW_MAX_LEN) {
-                let mut filled = 0;
-                while filled < s.len() {
-                    match fuchsia_zircon::cprng_draw(&mut s[filled..]) {
-                        Ok(actual) => filled += actual,
-                        Err(e) => panic!("cprng_draw failed: {:?}", e),
-                    };
-                }
-            }
+            fuchsia_cprng::cprng_draw(v);
         }
     }
 }